1. Field of the Invention
The present invention relates to a process for the separation of residues of toluenediisocyanate production, and more particularly to the separation of residues from mixtures obtained after reaction of toluenediamine and phosgene and preliminary separation thereof.
2. Description of the Prior Art
It is known from Ullmanns Encyklopadie der technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry] (1957), Vol. 9, pp. 8-10, that toluenediamine in the presence of an organic solvent having a boiling point lower than toluenediisocyanate, e.g., o-dichlorobenzene, is converted with phosgene into toluenediisocyanate. Generally, phosgenation is performed at low and high temperatures. As a rule, the reaction solution is then freed from residual phosgene by distillation or by entrainment with a stream of nitrogen and from the major amount of organic solvent by concentration in a distillation system. The reaction solution (concentrate) obtained usually contains 65-98 weight percent toluenediisocyanate, 0-10 weight percent lower-boiling organic solvent and 2-25 weight percent of nonvolatile residues. If higher-boiling organic solvents are used, for example, diethylisophthalate, it is advisable for the reaction and removal of residual phosgene to be followed by removal of toluenediisocyanate by distillation, resulting in a mixture (concentrate) of 70-98 weight percent of organic solvent, e.g., diethylisophthalate, 0-5 weight percent toluenediisocyanate and 2-25 weight percent of nonvolatile residues in the form of a solution or suspension.
The above-mentioned mixtures (concentrates) are now concentrated by distillation and the nonvolatile residues are thus separated from the fractions of the mixture of end-product and solvent, respectively, which are of interest in view of process economy, increase of end-product yield and reusable solvent. The last step consists of evaporation in customary evaporators, e.g., in residue pots in vacuum (Ullmann, loc. cit., top of p. 10).
Residues to be considered are urea derivatives, e.g., N,N'-di-(aminotoluyl) ureas, di-[(N-aminophenyl)ureido-(N')] toluenes, N,N'-di-(isocyanatotoluyl) ureas, di-[(N-isocyanatophenyl)-ureido-(N')] toluenes and corresponding biurets; uretdiones, e.g., N,N'-di-(aminotoluyl)-1,3-diazacyclobutane-2,4-dione, N,N'-di-(isocyanatotoluyl)-1,3-diazacyclobutane-2,4-dione; isocyanuric acid esters, e.g., tri-(aminotoluyl) isocyanurates, tri-(isocyanatotoluyl) isocyanurates; carbodiimides, e.g., di-(aminotoluyl) carbodiimides, di-(isocyanatotoluyl) carbodiimides. Since the mentioned compounds still contain active groups, such as amino- or isocyanate-groups, they can condense again, particularly during heating and/or in the presence of, e.g., fractions of starting diamine or phosgene, so that the residues will also contain corresponding di-, tri- or polyureas, di-, tri-, polyuretdiones, di-, tri-, polyisocyanuric acid esters, di-, tri-, polycarbodiimides, and in particular, polymers which simultaneously contain several ureide groups, biuret groups, carbodiimide groups and/or isocyanurate groups in the molecule. The compounds may be chlorinated in the ring and side chain. Depending on the solvent utilized, reaction products of toluenediamine or the mentioned compounds with decomposition products of the solvent may form, for example, the amide of formula ##STR1## from toluenediamine and decomposition products of diethylisophthalate.
In addition, metal chlorides may be present in the residue from corrosion processes. Generally, depending on the synthesis conditions, the residues contain 1-80 weight percent urea compounds, 0-40 weight percent uretdiones, 0-60 weight percent isocyanuric acid esters, 0.5-20 weight percent carbodiimides, and 5-95 weight percent more highly condensated or polymeric materials.
If such residues are heated in one step in the presence of active groups, as is the case, for example, during evaporation of the mentioned concentrates, continuous changes take place in the form of decomposition, chain rupture, ring scission and, particularly, further condensations. Such residues furthermore pass through high-viscosity phases during evaporation. On contact with hot wall surfaces or agitating equipment, this leads to caking and build-up. Deposits on the heating surfaces impair heat transfer. Very long drying times become necessary and evaporation is incomplete, while losses of solvent or toluenediisocyanate occur.
It is known from German Pat. No. 1,218,265 that materials in powder form can be mixed with helical agitators. It describes that products in the form of powder and fine granulate often tend to fuse or sinter during mixing and are unfavorably modified by the frictional heat generated during mixing. Such difficulties occur particularly with polymers, such as polyethylene or polypropylene. The individual particles fuse and sinter, inhibit the mixing motion, deposit on the agitator and with time, form large fused polymer coagulates. In such mixing processes, the product is forced downward in the center of the vessel and forced upward again on the vessel walls, and during deflection the mixed product is under high compressive and shear stresses at the vessel bottom. Such stresses increase the mentioned difficulties. The patent states (column 1) that products which tend to cake cannot be mixed by this method and recommends a specially designed helical agitator as the agitating equipment by means of which the powder materials are not caused to fuse or sinter by the generated heat of friction.